New research says invasive species of weeds, insects and worms are costing Africa more than $3.5tn (£2.5tn) every year.

Researchers based in Ghana, Kenya, Britain and Switzerland have highlighted the catastrophic effects of species introduced by human activity.

Nigeria, where losses are estimated at $1tn a year, is much the worst affected country.

The majority of the costs are from weeding - work primarily carried out by women and children - but damage caused by insects is estimated at almost $40bn.

The research authors noted that the findings may be underestimating the true cost of invasive species to Africa's agriculture - as they did not consider the costs of herbicides for disease and pest control.